Research On The Influence Of AC/DC Distribution Network Fault On Relay Protection And Solutions

In the modern society,the traditional AC distribution network faces a series of problems,such as the tension of line corridor,the insufficiency of power supply,the equipment breaking capacity can not meet the needs of large capacity,etc.But the AC/DC distribution network which contains the DC supply can solve all of these problems.It has better performance in conveying capacity,controllability,and the reliability of power supply and so on.The current research of DC distribution system is still in the preliminary stage.There are still a lot of works to do in the research of protection technology,structure design and fault diagnosis,etc.Therefore,it has great significance to study on the AC/DC distribution network.To solve its problems of fault and protection will be helpful to improve the reliability and construct the city distribution network.In this thesis,the typical double-end AC/DC distribution system is the research object.Firstly,the features of pole-to-ground fault in DC side are analyzed,and it occurs in the back-to-back system which adopt pole-to-ground mode.As for the fault process,it is divided into two stages: the DC capacitor discharging stage and the natural commutation to steady state.And the influence of the capacitance on the discharge current is also discussed.Secondly,in the back-to-back system with two poles,the single-phase ground fault which occurred at converter AC outlet is analyzed.The influences of the DC grounding mode,the parameters of capacitor and reactor on the fault are also discussed.Then the influence of system parameters on relay protection is further studied,and several conclusions are obtained: the longer the line is,the greater the filtering reactor is,the larger the internal resistance of the power supply is,the higher the rate of protection malfunction is.And based on the simulation software of DIgSILENT/PowerFactory,the above analyses are verified by simulation.Finally,based on the fault characteristics of both AC/DC sides,the protection of capacitor current change rate,low voltage protection of DC bus and the protection which adopts the superconducting current limiter are studied.